This application is based on Japanese Patent Applications No. 11-4241 filed Jan. 11, 1999, No. 11-123604 filed Apr. 30, 1999 and No. 11-184816 filed Jun. 30, 1999, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a braking system for an automotive vehicle, and more particularly to a braking system equipped with a booster and a master cylinder.
2. Discussion of the Related Art
JP-A-10-152041 discloses one example of a known braking system of the type indicated above. This braking system includes (a) a brake operating member such as a brake pedal, (b) a booster for boosting an operating force applied to the brake operating member, (c) a master cylinder including a housing, and a pressurizing piston which is substantially fluid-tightly and slidably received in the housing and which has opposite surfaces one of which cooperates with the housing to define a pressurizing chamber in front of said pressurizing piston, the pressurizing piston being advanced by an output force acting on the other of the opposite surfaces thereof, so that a fluid in the pressurizing chamber is pressurized, (d) a brake including a wheel brake cylinder which is connected to the pressurizing chamber of the master cylinder through a fluid passage and which is actuated by the pressurized fluid received from the pressurizing chamber, to apply a brake to a wheel of the vehicle, and (e) a pressure increasing device which is activated, when a predetermined pressure-increase initiating condition is satisfied, for increasing the pressure of the fluid in the wheel brake cylinder.
In this known braking system, the pressure increasing device is disposed in series with the booster, and can be considered to a second booster capable of performing an auxiliary boosting function. In this respect, the pressure increasing device may be designed such that the fluid pressure in the master cylinder is increased at a preset ratio. Where the pressure increasing device is designed according to this design concept, the overall ratio of a braking force produced by the wheel brake cylinder to the brake operating force acting on the brake operating member is expressed by a product of the boosting ratio of the booster and the pressure increasing ratio at which the fluid pressure in the master cylinder is increased by the pressure increasing device.
On the other hand, the operating characteristic or input-output characteristic of the booster tends to vary. This tendency is relatively high where the booster is a vacuum booster which uses a reduced or negative pressure source as a drive power source. The pressure of the negative pressure source available on a motor vehicle is comparatively difficult to accurately control such that the negative pressure is maintained at a predetermined level. For this reason, the operating characteristic of the vacuum booster has a high tendency of change or variation. Further, the operating characteristic of the booster has an influence on the characteristic of change in the fluid pressure in the master cylinder, that is, on the relationship between the brake operating force and the fluid pressure in the master cylinder. Accordingly, a change in the operating characteristic of the booster will result in a variation in the characteristic of change in the fluid pressure in the master cylinder.
Where the pressure increasing device is designed such that the fluid pressure in the master cylinder is increased at a preset ratio, the overall ratio of the braking force produced by the wheel brake cylinder to the brake operating force may deviate from the desired or nominal value to a comparatively large extent, if the characteristic of change in the master cylinder pressure varies with a change in the operating characteristic of the booster. This is because the amount of change in the operating characteristic of the booster is amplified by the predetermined pressure increasing ratio of the pressure increasing device which is connected in series with the booster. Thus, the known braking system has an undesirably high risk of deterioration of the operating feel of the brake operating member due to a change in the operating characteristic of the booster upon activation of the pressure increasing device.
It is therefore an object of the present invention to provide a vehicle braking system which is less likely to suffer from the deterioration of the operating feel of the brake operating member due to a change in the operating characteristic of the booster upon activation of the pressure increasing device.
The above object may be achieved according to any one of the following modes of the present invention, each of which is numbered like the appended claims and depends from the other mode or modes, where appropriate, so as to indicate and clarify possible combinations of elements or technical features. It is to be understood that the present invention is not limited to the technical features described below, or any combinations of those features.
(1) A braking system for an automotive vehicle having a wheel, comprising: a booster for boosting an operating force acting on a brake operating member; a master cylinder having a housing and including a pressurizing piston which is substantially fluid-tightly and slidably received in the housing and which cooperates with the housing to define a front pressurizing chamber and a rear pressurizing chamber on front and rear sides thereof, the pressurizing piston receiving at a rear end thereof an output force of the booster, a working fluid in the front pressurizing chamber being pressurized by an advancing movement of the pressurizing piston; a fluid passage connected to the front pressurizing chamber of the master cylinder; a brake including a wheel brake cylinder which is connected to the front pressurizing chamber through the fluid passage and which is activated by the fluid pressurized in the front pressurizing chamber, for braking the wheel; and a pressure increasing device activated when a predetermined pressure-increase initiating condition for initiating a pressure increase control is satisfied, to pressurize a fluid in the rear pressurizing chamber, for increasing a pressure of the fluid in the wheel brake cylinder to a value higher than a value before the predetermined pressure-increase initiating condition is satisfied.
In the braking system of the present invention constructed as described above, the pressurizing piston of the master cylinder is adapted to receive both the output force of the booster and a force based on the fluid pressure generated by the pressure increasing device. In the present arrangement, the pressure increasing device and the booster are disposed in parallel with each other, so that a sum of the output force of the booster and the force based on the fluid pressure generated by the pressure increasing device is applied to the wheel brake cylinder. Unlike the conventional braking system, the present braking system is free from a problem that a change in the operating characteristic of the booster is amplified by the pressure increasing device and the amplified change is transferred to the wheel brake cylinder. Thus, the present braking system is less likely to suffer from undesirable deterioration of the operating feel of the brake operating member due to the change in the operating characteristic of the booster.
In the present braking system wherein the electrically controlled pressure increasing device which is primarily controlled electrically and the booster which is primarily controlled mechanically are disposed in parallel with each other, the master cylinder can receive one of the output force of the booster and the force based on the fluid pressure generated by the pressure increasing device, even in the event of a failure of the booster or the pressure increasing device, unless both of the booster and the pressure increasing device simultaneously fail to normally function. Accordingly, the provision of the pressure increasing device provides an improvement in the operating reliability of the braking system, namely, an increased degree of fail-safe stability of the braking system.
The booster used in the present braking system may be a vacuum booster using a negative pressure source as its drive source, or a hydraulic booster using a hydraulic pressure source as its drive source.
The working fluid in the rear pressurizing chamber, which is pressurized by the pressure increasing device, may be either a liquid or a gas.
(2) A braking system according to the above mode (1), wherein the predetermined pressure-increase initiating condition is a predetermined operating condition of the booster in which a braking effect to be provided by the wheel brake cylinder is reduced, and the pressure increasing device is activated, upon detection of the predetermined operating condition, to pressurize the fluid in the rear pressurizing chamber of the master cylinder, so as to at least reduce an amount of reduction of the braking effect.
In the braking system according to the above mode (2), the amount of reduction of the braking effect due to a change of the operating condition of the booster is reduced by an operation of the pressure increasing device to apply a fluid pressure to the rear pressurizing chamber so that the pressurizing piston of the master cylinder receives at its rear end the fluid pressure.
In the present braking system, the predetermined pressure-increase initiating condition may be a condition that the boosting limit of the booster has been reached, so that the pressure increasing device is activated, upon detection that the boosting limit has been reached, to effect a braking effect characteristic control for reducing the amount of reduction of the rate of increase of the braking effect after the boosting limit has been reached. Alternatively, the pressure-increase initiating condition may be a condition that the booster fails to normally function, so that the pressure increasing device is activated, upon detection of a failure of the booster, to effect a pressure increase control for reducing the amount of reduction of the braking effect due to the failure of the booster.
The braking system according to the present invention may be adapted such that the brake indicated above is provided for each of a right wheel and a left wheel of the vehicle, and such that at least one of these two brakes for the right and left wheels is activated upon detection of an undesired yawing moment of the vehicle, so that there is produced a difference between the braking forces produced by the two brakes, for applying a yawing moment to the vehicle so as to offset the undesired yawing moment which would lower the running stability of the vehicle. In this case, the pressure-increase initiating condition may be the generation of the undesired yawing moment of the vehicle. Namely, the pressure increasing device may be activated upon detection of the generation of the undesired yawing moment of the vehicle, to apply one of the two brakes, for example, for increasing the running stability of the vehicle, without an operation of the brake operating member. Alternatively, the pressure-increase initiating condition may be a condition that the rate of operation of the brake operating member is higher than a predetermined upper limit, so that the pressure increasing device is activated when the brake operating member is rapidly operated, for providing an assisting braking force for abrupt brake application to the vehicle.
(3) A braking system according to the above mode (2), wherein the booster is a vacuum booster operated by a negative pressure source, and the predetermined pressure-increase initiating condition is a condition that a boosting limit of the vacuum booster has been reached, the pressure increasing device being activated when the boosting limit has been reached, for at least reducing an amount of reduction of a rate of increase of the braking effect after the boosting limit has been reached.
In the braking system according to the above mode (3), the amount of reduction of the rate of increase of the braking effect after the boosting limit of the vacuum booster has been reached is reduced by an operation of the pressure increasing device to apply a fluid pressure to the rear pressurizing chamber so that the pressurizing piston of the master cylinder receives at its rear end the fluid pressure.
(4) A braking system according to the above mode (3), wherein a pressure of the negative pressure source has a tendency of variation, and the operating force acting on the brake operating member when the boosting limit of the vacuum booster has been reached varies with the pressure of the negative pressure source, the predetermined pressure-increase initiating condition being a condition that the operating force acting on the brake operating member has increased to a boosting-limit brake operating force which is a value of the operating force to be detected when the boosting limit of the vacuum booster has been reached where an actual value of the pressure of the negative pressure source is equal to an upper limit of a variation range in which the actual value is expected to vary, the upper limit being a negative pressure value nearest to the atmospheric pressure.
In the braking system according to the above mode (4), the pressure increasing device is activated to pressurize the fluid in the rear pressurizing chamber, when the operating force of the brake operating member has increased to the boosting-limit brake operating force, which is to be detected when the boosting limit of the vacuum booster has been reached where the actual pressure of the negative pressure source is equal to the upper limit of the expected variation range of the pressure of the negative pressure source. Accordingly, the pressure increasing device is necessarily activated when the boosting limit of the vacuum booster has been reached, irrespective of a variation in the pressure of the negative pressure source.
The present braking system may use a sensor for detecting the actual value of the pressure of the negative pressure source or any related physical quantity which accurately represents the actual pressure value of the negative pressure source, to determine whether the pressure-increase initiating condition has been satisfied or not. Alternatively, the braking system may use a sensor for detecting any other physical quantity other than the actual pressure value and the above-indicated related physical quantity, which any other physical quantity changes with the actual pressure value and is more easily detectable than the above-indicated actual pressure value and related physical quantity. In the former case, the determination as to whether the boosting limit of the vacuum booster has been reached can be made on the basis of the detected actual operating force of the brake operating member as modified by the actually detected pressure of the negative pressure source, so that the determination can be made with comparatively high accuracy, without using the boosting-limit brake operating force as the threshold value of the operating force of the brake operating member which is to be detected when the boosting limit has been reached where the pressure of the negative pressure source is equal to the expected upper limit of the expected variation range. However, the use of the boosting-limit brake operating force as the threshold value is desirable for improved accuracy of the pressure increase control by the pressure increasing device, since there may be some variation in the detecting characteristic of the sensor provided to detect the actual pressure of the negative pressure source or the related physical quantity.
The braking system according to the above mode (4) is based on a fact that the boosting-limit brake operating force varies due to a variation of the actual value of the pressure of the negative pressure source toward and away from the atmospheric pressure, namely, due to a change in the operating characteristic of the vacuum booster due to the variation of the pressure of the negative pressure source. That is, the pressure increasing device is activated to initiate the pressure increase control, when the boosting limit of the vacuum booster has been reached where the pressure of the negative pressure source is equal to the expected highest value nearest to the atmospheric pressure. According to this arrangement, the pressure increasing device is activated before the boosting limit of the vacuum booster has not been actually reached, when the actual pressure of the negative pressure source is lower than the expected upper limit. In this case, the operation of the pressure increasing device to increase the fluid pressure in the wheel brake cylinder is performed while the booster is still normally operating to boost the brake operating force, so that the fluid pressure in the wheel brake cylinder may be higher than the desired value, when the actual pressure of the negative pressure source is lower than the expected upper limit, because the pressure increase control by the pressure increasing device is initiate before the boosting limit of the vacuum booster has been actually reached. However, the present braking system is free from the conventionally encountered problem that the variation of the operating characteristic of the booster due to a variation in the pressure of the negative pressure source is amplified by the pressure increasing device and the amplified variation is transferred to the wheel brake cylinder. Thus, the present braking system solves another problem that the pressure increasing device is not activated even after the boosting limit of the booster has been reached, while avoiding the problem of deteriorated operating feel of the brake operating member due to the wheel brake cylinder pressure which is higher than the desired value due to the variation in the pressure of the negative pressure source,
(5) A braking system according to any one of the above modes (1)-(4), wherein the master cylinder includes a second pressurizing piston in addition to the above-indicated pressurizing piston which serves as a first pressurizing piston, the first and second pressurizing pistons being disposed in series with each other in the housing such that the first pressurizing piston cooperates with the housing to define the above-indicated front pressurizing chamber as a first front pressurizing chamber while the second pressurizing piston cooperates with the housing to define a second front pressurizing chamber.
Where the conventional braking system described above is modified such that the master cylinder has two pressurizing pistons which are disposed in series with each other and which which cooperate with the housing to define respective two pressurizing chambers, two pressure increasing devices are provided downstream of the respective two pressurizing chambers to pressurize the fluid in the pressurizing chambers. In this modified conventional braking system, the fluid pressures in the two wheel brake cylinders connected to the two pressurizing chambers are desirably equal to each other, during operations of the two pressure increasing devices. To this end, the two pressure increasing devices are required to be controlled so as to increase the fluid pressures in the two wheel brake cylinders so that the fluid pressures in the two wheel brake cylinders are equal to each other.
In the braking system according to the above mode (5), on the other hand, the fluid pressures in the two front pressurizing chambers of the master cylinder are increased by the same amount by the fluid pressure in the rear pressurizing chamber, which is increased by the single pressure increasing device. Accordingly, the fluid pressures in the two wheel brake cylinders connected to the front pressurizing chambers are increased to the same level. Therefore, the present braking system does not require a special control of the pressure increasing device for equalizing the fluid pressures in the two wheel brake cylinders connected to the respective two front pressurizing chambers. In other words, the pressure increasing device can be comparatively easily controlled for establishing the same fluid pressure in the two wheel brake cylinders.
(6) A braking system according to any one of the above modes (1)-(5), wherein the pressure increasing device comprises (a) a physical quantity sensor for detecting a physical quantity which relates to the operating force acting on the brake operating member, (b) a pressure generating device electrically operated to pressurize the fluid in the rear pressurizing chamber; and (c) a controller for determining a desired value of the fluid pressure to be established in the rear pressurizing chamber, on the basis of the physical quantity detected by the sensor, and according to a predetermined relationship between the physical quantity and the desired value, the controller controlling the pressure generating device such that the fluid pressure in the rear pressurizing chamber is controlled to the desired value.
The physical quantity relating to the operating force of the brake operating member may be a physical quantity detected at a point upstream of the pressure increasing device as viewed in the direction in which the force is transmitted to the pressurizing piston. For instance, the physical quantity relating to the operating force may be the operating force of the brake operating member, or an amount of operation of the brake operating member. However, the physical quantity in question may be a physical quantity detected at a point downstream of the pressure increasing device, such as the fluid pressure in the front pressurizing chamber of the master cylinder, or the fluid pressure in the wheel brake cylinder. For establishing the desired fluid pressure in the wheel brake cylinder by operation of the pressure increasing device, it is desirable that the physical quantity relating to the operating force of the brake operating member be a physical quantity detected at a point upstream of the pressure increasing member, which physical quantity is not influenced by an increase in the fluid pressure in the front pressurizing chamber by the pressure increasing device. However, there exists a predetermined relationship between a physical quantity detected upstream of the pressure increasing device and a physical quantity detected downstream of the pressure increasing device, so that the physical quantity detected downstream of the pressure increasing device may be used to estimate the physical quantity detected upstream of the pressure increasing device.
(7) A braking system according to the above mode (6), wherein the rear pressurizing chamber is filled with a working liquid as the working fluid, and the pressure increasing device further comprises a flow control device having a non-operated state in which the flow control device permits flows of the working liquid into and from the rear pressurizing chamber, and an operated state in which the flow control device inhibits at least a flow of the working liquid from the rear pressurizing chamber.
The volume of the rear pressurizing chamber is increased as the brake operating member is operated in a direction for increasing the operating force. On the other hand, the volume of the rear pressurizing chamber is reduced as the brake operating member is operated in a direction for reducing the operating force. Where the rear pressurizing chamber is filled with a non-compressible working liquid as the working fluid, the flows of the working liquid into and from the rear pressurizing chamber must be permitted during an operation of the brake operating member without an operation of the pressure increasing device, and the flow of the working liquid from the rear pressurizing chamber must be inhibited during an operation of the pressure increasing device.
In the braking system according to the above mode (7) wherein the flows of the liquid into and from the rear pressurizing chamber are permitted when the flow control device is in the non-operated state, and at least the flow of the liquid from the rear pressurizing chamber is inhibited when the flow control device is in the operated state. The flow control device is normally placed in the non-operated state without an electric energy being applied thereto, for permitting the working fluid to flow into and from the rear pressurizing chamber, so that the brake operating member is permitted to be operated in the opposite directions. When the pressure increasing device is activated, the flow control valve is brought into the operated position with an electric energy applied thereto, for inhibiting the working liquid from being discharged from the rear pressurizing chamber, so that the fluid pressure in the rear pressurizing chamber can be increased by the pressure increasing device.
(8) A braking system according to any one of the above modes (1)-(7), wherein the pressure increasing device includes a hydraulic pump whose output is connected to the rear pressurizing chamber, for pressurizing the fluid in the rear pressurizing chamber.
(9) A braking system according to any one of the above modes (1)-(7), wherein the pressure increasing device includes a hydraulic pump whose output is transmitted to the rear pressurizing chamber, without using an accumulator, for pressurizing the fluid in the rear pressurizing chamber.
In the braking system according to the above mode (8), the pressure increasing device does not include an accumulator, for pressurizing the fluid in the rear pressurizing chamber, so that the required size and weight of the pressure increasing device can be reduced.
The hydraulic pump in the braking system according to the above mode (8) may be of a plunger type having a high degree of tightness with respect to the working fluid, or of a gear type whose delivery or output pressure has a relatively small amount of pulsation.
(10) A braking system according to the above mode (8) or (9), wherein the front pressurizing chamber of the master cylinder and the wheel brake cylinder are connected to each other such that the front pressurizing chamber and the wheel brake cylinder are not held disconnected from each other during an operation of the hydraulic pump.
In the braking system according to the above mode (10) wherein the pressure pulsation of the pressurized fluid delivered by the hydraulic pump is transmitted to the pressurizing piston of the master cylinder through the rear pressurizing chamber, the pressure pulsation transmitted to the pressurizing piston is transmitted to the wheel brake cylinder through the front pressurizing chamber and the fluid passage, since the front pressurizing chamber and the wheel brake cylinder are not held disconnected during the operation of the hydraulic pump. Since the wheel brake cylinder provides a damping effect for attenuating or absorbing the pressure pulsation, the amount of transmission of the pressure pulsation of the hydraulic pump to the brake operating member is effectively reduced by the damping effect of the wheel brake cylinder, so as to reduce the deterioration of the operating feel of the brake operating member.
(11) A braking system according to the above mode (10), wherein the fluid passage is provided with a pressure control valve for controlling the fluid pressure in the wheel brake cylinder, the front pressurizing chamber of the master cylinder being connected to the pressure control valve without a cut-off valve being interposed therebetween.
The braking system according to the above mode (11) is a preferred arrangement of the above mode (10).
(12) A braking system according to the above mode (6) or (7), wherein the pressure generating device includes (a) a hydraulic cylinder having a housing, and a control piston substantially fluid-tightly and slidably received in the housing and cooperating with the housing to define a control pressure chamber which is connected to the rear pressurizing chamber and whose fluid pressure is increased by an advancing movement of the control piston, (b) an electric motor, and (c) a motion converting mechanism for converting a rotary motion of the electric motor into a linear motion of the control piston, and wherein the controller controls the electric motor so as to control an operating stroke of the control piston, for thereby regulating a pressure of the fluid in the control pressure chamber.
In the braking system according to the above mode (12), the pressure generating device having a comparatively simple construction is capable of generating a fluid pressure in the rear pressurizing chamber, and regulating this fluid pressure with the electric motor being controlled by the controller.
The motion converting mechanism of the pressure generating device may include an externally threaded member which is supported such that the externally threaded member is axially movable but is not rotatable, and an internally threaded member or a nut which engages the externally threaded member and is supported such that the internally threaded member is rotatable but is not axially movable. In this instance, the internally threaded member is rotated by the electric motor in a selected one of forward or reverse directions, to advance or retract the control piston with the externally threaded member. Alternatively, the externally and internally threaded members may be supported such that the externally supported member is rotatable but is not axially movable and is rotated by the electric motor and such that the internally supported member is not rotatable but is axially movable with the control piston.
(13) A braking system according to the above mode (12), wherein the rear pressurizing chamber is filled with a working liquid as the working fluid, and the pressure increasing device further comprises a reservoir storing the working liquid, and wherein the housing of the hydraulic cylinder of the pressure generating device has a control port connected to the reservoir, the control port being position such that the control port communicates with the control pressure chamber when the control piston is placed in a fully retracted initial position thereof, and does not communicate with the control pressure chamber after the control piston has been advanced a predetermined distance from the initial position.
In the braking system according to the above mode (13), the fluid pressure in the control pressure chamber can be raised by an advancing movement of the control piston during an operation of the pressure increasing device, since the control port connected to the reservoir is closed by the advanced control piston. While the pressure increasing device is not in operation with the control piston placed at its fully retracted initial position, the control port permits fluid communication between the control pressure chamber and the reservoir, and therefore flows of the working liquid therebetween in the opposite directions. Thus, the pressure generating device cooperates with the controller to permit an increase in the liquid pressure in the control pressure chamber to pressurize the liquid in the rear pressurizing chamber of the master cylinder during an operation of the pressure increasing device. In a normal braking operation with an operation of the brake operating member without an operation of the pressure increasing device, on the other hand, the pressure generating device permits the liquid to flow in the opposite directions between the reservoir and the rear pressurizing chamber through the control pressure chamber and the control port, so as to permit a change of the volume of the rear pressurizing chamber depending upon the operation of the brake operating member. The pressure generating device having these two functions contributes to simpler construction of the pressure increasing device as a whole.
(14) A braking system according to any one of the above modes (6)-(13), wherein the rear pressurizing chamber is filled with a working liquid as the working fluid, and the pressure increasing device further comprises (a) a reservoir storing the working liquid, (b) a by-pass passage which by-passes the pressure generating device and which is connected at one of opposite ends thereof to the rear pressurizing chamber and at the other end to the reservoir, and (c) a check valve provided in the by-pass passage and which permits a flow of the liquid in a first direction from the reservoir toward the rear pressurizing chamber, and inhibits a flow of the liquid in a second direction opposite to the first direction.
In the braking system according to the above mode (14), the by-pass passage and the check valve always permit the working liquid to flow from the reservoir into the rear pressurizing chamber of the master cylinder, while by-passing the pressure generating device. In this arrangement, a rapid operation of the brake operating member in a direction for increasing the brake operating force will cause the liquid to be supplied from the reservoir to the rear pressurizing chamber, not only through the pressure generating device, but also through the by-pass passage and the check valve, so as to prevent development of a negative pressure in the rear pressurizing chamber, which would take place due to a low rate of supply flow of the liquid from the reservoir into the rear pressurizing chamber upon a rapid increase of the volume of the rear pressurizing chamber caused by a rapid operation of the brake operating member.
(15) A braking system according to any one of the above modes (6)-(14), wherein the rear pressurizing chamber is filled with a working liquid as the working fluid, and the pressure increasing device further comprises (a) a communication passage connected to opposite ends thereof to the front pressurizing chamber and the rear pressurizing chamber, respectively, (b) a solenoid-operated shut-off valve provided in the communication passage and having an open state for permitting flows of the liquid therethrough and a closed state for inhibiting the flows of the liquid therethrough, and (c) a controller for placing the shut-off valve in the open state when the pressure generating device is not normally operable to pressurize the liquid in the rear pressurizing chamber, and in the closed state when the pressure generating device is normally operable.
In the braking system according to the above mode (15), the liquid in the rear pressurizing chamber can be pressurized by the liquid pressure in the front pressurizing chamber through the communication passage, during an operation of the brake operating member, even in the event of a failure of the pressure generating device, unless the booster is defective.
(16) A braking system according to the above mode (15), wherein the pressure increasing device further comprises a flow control device which inhibits a flow of the liquid in a direction from the front pressurizing chamber toward the rear pressurizing chamber when a pressure of the liquid in the front pressurizing chamber is not higher than a predetermined level, and permits the flow of the liquid in the direction from the front pressurizing chamber toward the rear pressurizing chamber when the pressure in the front pressurizing chamber is higher than the predetermined level.
If the braking system according to the above mode (15) is not provided with the flow control device provided in the braking system according to the above mode (16), and if the shut-off valve provided in the communication passage is opened immediately after a failure of the pressure increasing device is detected after the predetermined pressure-increase initiating condition is satisfied, the liquid pressurized in the front pressurizing chamber by the output force of the booster is delivered not only to the wheel brake cylinder but also to the rear pressurizing chamber immediately after the failure of the pressure increasing device is detected. In this situation, the liquid pressure in the wheel brake cylinder cannot be increased rapidly by the delivery thereto of the pressurized liquid from the rear wheel brake cylinder immediately after the failure of the pressure increasing device is detected, since the shut-off valve is opened immediately after the detection of this failure. This tendency is relatively high where the brake operating member is rapidly operated, resulting in a rapid increase of the brake operating force, which in turn causes the predetermined pressure-increase initiating condition to be satisfied in a short time after the initiation of the operation of the brake operating member.
In the braking system according to the above mode (16) wherein the flow control device is provided, the flow of the liquid from the front pressurizing chamber toward the rear pressurizing chamber is inhibited by the flow control device while the liquid pressure in the front pressurizing chamber is lower than the predetermined level, so that the liquid pressure in the wheel brake cylinder can be rapidly increased. Thus, the operating feel of the brake operating member will not be deteriorated. Further, the flow control device permits the liquid pressure in the rear pressurizing chamber to be increased by the liquid pressure in the front pressurizing chamber after the latter liquid pressure has exceeded the predetermined level.
The flow control device may or may not utilize the shut-off valve provided in the communication passage described above.
(17) A braking system according to the above mode (16), wherein the flow control device includes a first check valve provided in the communication passage, the first check valve inhibiting the flow of the liquid from the front pressurizing chamber toward the rear pressurizing chamber when the pressure of the liquid in the front pressurizing chamber is not higher than the pressure of the liquid in the rear pressurizing chamber by more than a predetermined opening pressure difference which is not substantially zero, the first check valve permitting the flow of the liquid from the front pressurizing chamber toward the rear pressurizing chamber when the pressure of the liquid in the front pressurizing chamber is higher than the pressure of the liquid in the rear pressurizing chamber by more than the predetermined opening pressure difference.
In the braking system according to the above mode (17), the flow control valve has a comparatively simple construction which does not use an electric energy.
(18) A braking system according to the above mode (17), wherein the pressure increasing device further comprises (a) a by-pass passage which by-passes the first check valve, and (b) a second check valve provided in the by-pass passage, the second check valve permitting a flow of the liquid in a first direction from the rear pressurizing chamber toward the front pressurizing chamber and inhibiting a flow of the liquid in a second direction opposite to the first direction, irrespective of a difference between the liquid pressures in the front and rear pressurizing chambers.
In the braking system according to the above mode (18), the liquid can be supplied from a suitable reservoir to the front pressurizing chamber through the second check valve and the communication passage, since the second check valve is provided in the by-pass passage which by-passes the first check valve that inhibits the liquid flow into the front pressurizing chamber. This arrangement is effective to prevent the liquid pressure from being lowered below the atmospheric pressure even when the volume of the front pressurizing chamber is rapidly increased upon rapid releasing of the brake operating member toward its non-operated position.
(19) A braking system according to any one of claims 1-5, wherein the rear pressurizing chamber is filled with a working liquid as the working fluid, and the pressure increasing device comprises (a) a communication passage connected at opposite ends thereof to the front and rear pressurizing chambers, respectively, (b) a solenoid-operated shut-off valve provided in the communication passage, and (c) a controller for controlling the shut-off valve such that the shut-off valve is open while the pressure-increase initiating condition is satisfied, and closed while the pressure-increase initiating condition is not satisfied.
In the braking system according to the above mode (19), the liquid in the rear pressurizing chamber can be pressurized by the liquid pressure in the front pressurizing chamber, which is pressurized by the output force of the booster during an operation of the brake operating member, unless the booster is defective. This arrangement does not require the pressure increasing device to be a pressure generating device independent of the master cylinder, in order for the pressure increasing device to pressurize the liquid in the rear pressurizing chamber.
The feature of the above mode (19) is available in combination of any one of the features of the above modes (16)-(18).
(20) A braking system according to any one of the above modes (1)-(19), wherein the pressure increasing device comprises a vacuum pump which is operated by a negative pressure supplied from a negative pressure source, to pressurized the working fluid in the rear pressurizing chamber.
In the braking system according to the mode (1) described above, the pressure increasing device may include a hydraulic pump which is operated by an electric motor, to pressurize the fluid in the rear pressurizing chamber. The braking system according to the above mode (20) is advantageous over the braking system wherein the pressure increasing device uses a hydraulic pump and an electric motor, in that the braking system according to the above mode (20) does not use an electric motor and that the negative pressure source is usually easily available on an automotive vehicle, without a special mechanism. Namely, the braking system according to the above mode (20) wherein the pressure increasing device uses a vacuum pump is comparatively economical to manufacture, and has reduced weight and operating noise and improved operating reliability.
(21) A braking system according to the above mode (20), wherein the booster is a vacuum booster which is operated by the negative pressure supplied from the negative pressure source.
In the braking system according to the above mode (21), the negative pressure source is used for operating not only the booster in the form of the vacuum booster but also the vacuum pump of the pressure increasing device, whereby the braking system can be made simpler in construction with a reduced number of components.
(22) A braking system according to the above mode (20) or (21), wherein the vacuum pump includes (a) a housing, (b) a plunger slidably received within the housing and cooperating with the housing to define a pump chamber connected to the rear pressurizing chamber, and (c) a diaphragm slidably received within the housing and cooperating with the housing to define a variable-pressure chamber and an atmospheric pressure chamber on opposite sides of the diaphragm, the variable-pressure chamber being selectively exposed to an atmospheric pressure and the negative pressure of the negative pressure source, while the atmospheric pressure chamber being held exposed to the atmospheric pressure, the plunger being moved with the diaphragm depending upon a difference between pressures in the variable-pressure chamber and the atmospheric pressure chamber, so that a volume of the pump chamber changes with a movement of the plunger.
(23) A braking system according to any one of the above modes (20)-(22), wherein the vacuum pump further includes a three-way valve connected to an atmosphere, the negative pressure source and the variable-pressure chamber, the three-way valve having a first state in which the variable-pressure chamber is disconnected from the negative pressure source and is communicated with the atmosphere, and a second state in which the variable-pressure chamber is disconnected from the atmosphere and is communicated with the negative pressure source.
In the braking system according to the above mode (23), the atmospheric pressure and the negative pressure are selectively applied to the variable-pressure chamber of the vacuum pump, through the single three-way valve. Thus, the pressure increasing device does not require two control valves one for applying the atmospheric pressure and the other for applying the negative pressure. Accordingly, the valve arrangement for selectively applying the atmospheric pressure and the negative pressure to the variable-pressure chamber of the vacuum pump can be simplified, so that the vacuum pump as a whole can be simplified in construction.